All the tasks assigned to the adapters are not equally urgent and, therefore, these tasks are given priority levels. Handling of the adapters is relatively simple when each of them is given tasks having the same level of priority. It then suffices to determine the identities of those adapters which require service to know which of them must be serviced first.
It is, however, not always possible to proceed this way since the demands of the teleprocessing system architecture often require that tasks with different priority levels be assigned to a single adapter. The adapters then can be distributed into groups, and each group can be provided with an auto-selection system. Before requesting service or servicing an order coming from the CCU, the adapters in a single group proceed to execute a selection amongst them in order to determine which adapter will be activated. However, this auto-selection operation is not carried out instantaneously which entails a waste of time detrimental to the controller throughput.